1. Field of the Invention
The present invention relates to a method of producing a welded tube with an excellent corrosion-resistant inner surface, in which the inner surface as plated with a metal or alloy has excellent corrosion resistance to be free from corrosion especially that from the welded part (hereinafter referred to as a bead part).
2. Description of the Prior Arts
Heretofore, a small-diameter steel tube has been used as a fuel pipe for cars. As pipes for lower gasoline, sour gasoline, alcohol and alcohol-mixed fuels, formation of an anti-rusting metal layer on the inner surface thereof for improving the rust resistance of it has been proposed. For instance, there has been proposed a method of forming a fused film on the inner surface of a metal tube; in which a film forming material of tin, lead, zinc or an alloy based on the metals is inserted into the inside of a long-size metal tube, the both edges of the metal tube are crushed with leaving a slight air-passing slit therethrough, and the tube is heat-treated at a temperature of from 700.degree. to 1150.degree. C. by passing it through a continuous heating furnace having a non-oxidizing gas atmosphere therein under the condition to thereby form a film over the inner surface of the metal tube by fusion coating (Japanese Patent Publication No. 58-49636).
However, since the proposed method is one to coat one single metal layer or alloy layer on the inner surface of a tube, it has a problem that pin holes or spots not well coated with the layer with due to coating failure, if any, on the coated surface would often be corroded by water to be in alcohol or alcohol-mixed fuel or by organic acids to be formed by oxidation of gasoline or decomposition of alcohol.